1. Field of the Invention
The invention, in general, relates to a method of fabricating organically bound wood-based materials and, more particularly, to a method of fabricating boards from particles such as chips and fibers containing lignocellulose and organic binding materials. Such boards are called, and will hereinafter sometimes be referred to as, particle boards.
2. Background of the Invention
Wood-based materials made from fibers, chips or strands may be fabricated particularly economically using organic binding materials. Condensation resins, such as, for instance, urea- and phenol-formaldehyde and isocyanates, including mixtures thereof, are most often used for agglutination. The hardening or curing time of organic binders is a function of temperature as well as of time. For reasons of economy or efficiency the fibrous webs made from agglutinated particles are usually compressed in heat presses. Aside from other factors such as the type of binder used, the shape of the particles, and the structure of the fibrous web, the compression or molding time is decisively determined by the time required for heating the fibrous web to the gel point or gelation temperature of the binder. Since the cost-efficiency of the fabrication of wood-based materials is significantly influenced by the required compression time t.sub.p, it is desirable that heat penetrate into the fibrous web in the shortest possible time. Heat is transferred from the heat press plates to the fibrous web by heat conduction and convection. High densities result in high fluid resistances and low moisture detrimentally affects heat conduction. Yet mechanical specifications of the wood-based materials call for board densities at which the high fluid resistance significantly affects the compression time. However, it is not expedient to compensate for this negative influence of the density by a higher moisture content of the particles in the entire chip mass. While it is possible to reduce the time required for heat penetration, the overall press time is not reduced because it is necessary for steam to escape by way of the narrow surfaces of the board.
Several methods have been proposed and used in practical applications. Thus, DE F1658 XII/381 proposes to vary the moisture in the individual layers of a board or web. Heat penetration is enhanced if the cover layers are significantly moister than the center layer. The moisture of the particles has, however, to be selected such that the quantity of steam convected into the center layer leads to more rapid heating without, at the same time, creating an internal pressure which would destroy the glue joints between particles of the center layer when the press is opened, by rupturing those joints. The steam energy useful for heat penetration is, therefore, limited.
Selective through-heating by steam flowing form the moist cover layers into the center layer constitutes the physical basis of the steam injection process developed by Klauditz. In this process, the cover layers of a fibrous chip web are sprayed with water. In respect of through-heating spraying water onto the cover layers (moistening) has been found to be superior to using chips of increased moisture contents. The evaporation of water permeating the wood structure as moisture requires more energy than the water deposited on chip surfaces (Kollmann, HRW 1957, pp. 35-44; Keylwerth, Holzforschung und Holzverwertung 1959, pp. 51-57; Stegmann, v. Bismark, Holzforschung und Holzverwertung 1967, pp. 53-59).
The time required to reach the boiling point in the center of the fibrous web depends upon the temperature of the heating plates and upon the quantity of water m.sub.w sprayed onto a given surface area. Several experiments have shown that beyond a certain quantity of water the relationship between the applied quantity of water and the time required for through-heating regresses. For practical purposes, quantities of water in excess of 200 g/m.sup.2 with chip boards of 20 mm thickness are not expedient. Moreover, large sprayed-on quantities of water result in reduced flexural and peeling strengths. High heating plate temperatures improve rapid evaporation of the sprayed-on water, and the time required for through-heating is lessened.
A further process of reducing the press time has been described by Pungs and Lamberts (HRW 1954, pp. 20-25). In accordance with this process, a particle board web is heated by high frequency heating rather than by heating plates. The advantage of such a process is uniform heating throughout the entire thickness of the board. Its disadvantage is its high consumption of electrical energy. Nor has pre-heating of fibrous webs by high frequency found any acceptance notwithstanding the fact that in a chip board of 20 mm thickness, pre-heating from 20.degree. C. to 60.degree. C. resulted in a press time reduction from 160 seconds to 90 seconds. Aside from its high energy consumption, a further and serious disadvantage of this process are the difficulties of shielding the fiber board support from adjacent support components and to structure it without using metal.
The through-heating of fibrous webs by means of steam is described in U.S. Pat. No. 4,517,147. In accordance with the system there disclosed, the press plates are perforated, and during closure of the press, steam may be blown into the web over the surface of the board. The released condensation heat leads to a rapid through-heating of the web. The complexity of the apparatus is at present justified only in connection with thick boards and with insulating webs which because of their low density have poor heat conducting properties.
The heretofore known methods of rapidly through-heating chip or fiber board webs require particularly complex equipment where press times are desired which are shorter than those of Klauditz's steam injection method.